Simple JFET Buffer Preamp

[Johnloudb]

I had this box that needed a small circuit and one that would work with the +/-12volt battery supply. It used to be an EVS passive preamp. So, I put a JFET buffer similar to Nelson Pass' B1 Buffer in it.





It uses a dual rail supply which allows for the elimination of the coupling caps. I put a trim pot at the output to adjust DC offset. If I want I could eliminate the pot in the future by replacing it with a 10 ohm resistor paralleled with two larger value resistors that form a voltage divider.

Also, to make the DC offset pot work at the output you need to measure the Idss of the JFETs and have the lower Idss JFET as the current source, otherwise the other JFET is biased higher that Idss, with a gate to source voltage that is positive. This is a problem because it results in a negative DC offset, and the potentiometer can only make the output DC voltage less positive, not more positive.



Anyway, there it is. Oh yeah, the 1k gate stopper on the current source is not there. I did the current source just like Pass' B1, with the gate shorted to the source.

One reason I wanted to do this is because it will be the building block for my discrete JFET crossover for my dipole speaker project. So, thought I would give it a listen in our current system.

[Johnloudb]

Each board also includes a cap multiplier that allows for adjustment of the supply voltage.

[Hank]

SO, this is what's keeping you from making sawdust! Who's JFET's? How does it "sound"?

[JonMarsh]

+1

SO, this is what's keeping you from making sawdust! Who's JFET's? How does it "sound"?

What Hank says...

I've just boarded a UA flight in Raleigh to head back to CA for the weekend- hope to make a little sawdust this weekend, though chances are I'll mostly be doing laundry and cleaning bathrooms! Though maybe a barbecue Sunday afternoon, for Beverly's niece and boyfriend who are visiting. Not a lot of summer left, and I've hardly gotten started on it!

I'll go one further than Hank and ask for square wave photos and distortion plots while you're at it!

[Johnloudb]

SO, this is what's keeping you from making sawdust! Who's JFET's? How does it "sound"?

Not too much Hank. Like most people I have other issues that keep me from getting stuff done. Also, I've been learning a lot as I go with the wood working, veneer and stuff. I'm kind of lost when it comes to building a speaker cabinet and was kind of stumped on how to go about the supports, paint, veneer and stuff. But, I got it pretty much figured out what I want now. Got to get to finish CADing up the supports this weekend. Then Monday I'll contact the CNC shop again and see what he wants.

Who's JFET's? I got these from Pass Labs DIY years ago. Actually, I don't have anymore so might go with the LSK170B for the crossover. It's basically the same JFET as the 2sk170bl. Maybe try the BF862, but it would have to be biased differently as it has a higher Idss. The ones I have are like 17mA and would have to be biased lower than that. It's supposed to be a good one with lower noise, higher transconductance, low capacitance.

About the sound I'm hooking these up tonight to our main system, so I will have listening impressions soon. I've been burning this in for about 5 days with a tuner. It sounds good best I can tell with my T-amp mini system on my workbench.

[Johnloudb]

What Hank says...

I've just boarded a UA flight in Raleigh to head back to CA for the weekend- hope to make a little sawdust this weekend, though chances are I'll mostly be doing laundry and cleaning bathrooms! Though maybe a barbecue Sunday afternoon, for Beverly's niece and boyfriend who are visiting. Not a lot of summer left, and I've hardly gotten started on it!

I'll go one further than Hank and ask for square wave photos and distortion plots while you're at it!

Jon, what have you hardly gotten started on? Is that the Ardent with the Scan woofers or something else? The barbecue sounds nice!

Here's the square wave response, 10kHz - not bad. It actually produces a beautiful 100kHz square wave at 100kHz without the volume control, but I used a 20k resistor with a 100k shunt volume control. I'm kind of second guessing that now and I may just put in a 20k noble pot I have, but I'll give this a listen first. It will likely be played near max volume.



As for distortion, I haven't looked at that as I suspect I'd just see the distortion and noise of my soundcard.

[Hank]

Lookin' good. We need to get Mark H to chime in here with an update re his active crossover plans. John, what city are you in?

[BigJim_inFLA]

If you wanted to stick with Toshiba 2sk170bl I have about a hundred of them lying around here somewhere. They are leftovers from this little project...



It is a Pass design also, JfetBOZ preamp, runs on 19v and gives about 24 dB of gain. The sound is quite nice! Anyway, PM if you want some of the 2sk170bl.

[Johnloudb]

Lookin' good. We need to get Mark H to chime in here with an update re his active crossover plans. John, what city are you in?

Sure, I welcome Mark's input on the crossover design. I haven't seen him on the board for awhile though. He likes op amps and I plan on going the discrete JFET route. Though, once I take some measurements will most likely need some pointers to get things right. I'm in Salt Lake City.

I have a Beringer DCX2496 that I plan to use at first to experiment with different crossover points and slopes, like Chuck did with his speaker.

I listened to this preamp last night and it was very good, but not quite up to my last preamp design. Maybe if I pimp out this buffer more with larger supply caps and change the volume pot it will sound better.

Big Jim,

Thanks very much for the offer. Just may take you up on that.

[Johnloudb]

Well, my dad listened to this thing this morning ... and liked it but in the end he came to the same initial conclusion that I did, that it was a bit closed in sounding and didn't have the dynamics and bass definition of my previous preamp. It does have nice detail and is pleasant though. I think the caps in the multiplier are suspect. I just used what I on hand.

So, I will order some Panasonic FM caps, 16v, 680uF to replace the 22uF caps.
And maybe another 47uF FM cap to replace the old one, and add some bi-pass caps.

And change the volume control to a non shunted 20k.

Oh and the gel cell batteries have a regulator at the output, that regulates the voltage to 11v.

If someone should try this, one might be better of just using regulators (one for each channel) to get the +/- 9 volts, and forget the multiplier. I built this battery supply with some other circuit in mind.

The battery has a charging circuit of my design as well, but I can't remember exactly the component values and stuff. There are commercial chargers out there of course ...

[Hank]

Simplify - out with the multipliers. I'll PM or call Mark and try to get him back here to espouse his opamp proclivities.

[Johnloudb]

If I take out the multipliers then I have to built shunt regulators.

Anyway, I think with the changes I'm making it will sound much better, not that it sounded that bad. I'm also going to try 500 ohms at the output which won't be so much impedance. The little buffer has to drive 10k ohm amp impedance plus the Behringer crossover which controls the woofers and subwoofers. Just ordered parts.

The supply for the active crossover will be different.

[Johnloudb]

I finished my modifications on the preamp, and it now traces a very pretty 100kHz square wave.

As for the sound, it is a bit better, with more air and detail.

But, the real problem seems to be with the Behringer crossover. With this preamp in the mix I have to turn down the crossover output volume 6dB cause my other preamp had a ballanced output going to the high frequency amps, and a single ended output to the crossover.

Anyway, before I did this mod I turned the input down on the crossover, and that is what made it sound closed in. I first did the same thing with the modded buffer preamp and was disapointed in the results. But, when I turned the volume down on the output instead of the input of the crossover, the sound was significantly better.

My old preamp, is still better sounding, slightly less detail, but much more dynamic with more bass slam and bigger soundstage. I think this could be just because of the Behringer crossover and the fact that turning down the digital volume controls costs you dynamic range.

So, I've got to add volume controls to the Behringer output, so I can turn down the output without loosing dynamic range.

Does anyone know of an add on commercial unit that does this?

[Davey]

http://www.sound4sale.com/products/MVC.php

Or, you could incorporate a multi-channel volume control within the Behringer crossover itself. See Jan Didden's effort.

Dave.

[Johnloudb]

Thanks Davey,

That looks nice.

Where can I find Jan Didden's effort?

[cjd]

Or, you could replace the Behringer. I've not ever heard one that really did justice to anything with solid low end OR any layered complexity (i.e. large symphonic works) and that's WITH a very talented designer and high-end drivers.

[Johnloudb]

Hi Chris,

Yeah, I should probably have a better crossover. It's just nice to have the flexibility the Behringer offers. Other crossovers all seem to be LR4 types and our sub sounds much better integrated with a 4th order Butterworth at 86Hz. It's not in the signal path above 300Hz right now.

Anyway, I may put a subwoofer crossover circuit in my active crossover circuit.

[Hank]

Yeah, the Behringer is not top shelf audio. I remember seeing a French company advertising replacement circuits for at least one Behringer unit in "Audio Xpress" mag.

Now about your DLA project...

[Johnloudb]

having an ISP Nightmare!!!
Will be back online after 09/14
Hank, DLA coming along